Fig. 1: Formation of double helix by Bis(indole) compounds.

Chemical structures of bis(indole) molecules 1a−1e with single and double-stranded helical schematic representation (a). The side view of the X-ray crystal structures of 1a (b) and 1b (c) forming double helix structures. Two helices in each double helix structure are shown in two different colors, where hydrogen bonds are shown as dotted lines, and the cyan rods along the helical axis are added for a better understanding of the double helix. The solvent molecules in the crystal structure of 1a and 1b were omitted for clarity. The partial ¹H−¹H NOESY NMR spectrum of a mixture of 1c (3.0 mM) and 1d (3.0 mM) in CDCl₃ showing the H_g and H_g’ interaction from a hetero-dimeric double helix structure (d). The partially stacked spectra of ¹H NMR dilution experiment of 1c in CDCl₃ at 25 °C (e). Chemical shifts of N−H protons in DMSO-d₆ titration of 1c (5.0 mM) in CDCl₃ at 25 °C where the spectra were calibrated by tetramethylsilane (TMS) signal (f). The partially stacked ¹H NMR spectra of 1c (0.1 mM) in C₂D₂Cl₄ recorded at variable temperature (g).